ElectricCircuitTheoryHW4Fa09 - EK307 Electric Circuit...

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EK307 – Electric Circuit Theory – Homework #4 – Fall 2009 Tutorial Problems on Transistors and Diodes Problem 1 ) The semiconductor diode (sometimes called the pn -junction diode) is a very common non-linear circuit element. The equation for the diode current due to an applied voltage is exponential in form: ) 1 ( / - = T D V v S D e I i Here I S is a device-specific constant called the scale current , and V T is the thermal voltage given by kT / q . (1) Note that V T = 0.025 V at room temperature. This equation is definitely not linear and does not have the form y = mx + b . (Contrast to Ohm’s Law: i = v / R. ) In this problem, we plot the current i D as a function of applied voltage v D . a) For a diode having I S = 10 - 12 A, fill in the table to the right, showing the current i D that results for each value of v D . b) Now plot these numbers with the i D values from the table on the vertical axis, and the v D values from the table on the horizontal axis. The v D axis itself should extend from zero to 5 V. What letter of the alphabet or geometrical form best described the nature of this curve? c) We define the value v D = V f as the point where the curve makes its transition from being essentially horizontal to being essentially vertical. The voltage V f represents the “threshold” value of v D below which current will not flow. Estimate the value of V f for this diode. Problem 2 ) As in Prob. 1, estimate the value of V f for diodes having scale currents of a) I s = 10 - 15 A b) I s = 10 - 10 A Problem 3 ) A diode for which V f = 0.7 V is connected to a Thévenin circuit, as shown below.
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ElectricCircuitTheoryHW4Fa09 - EK307 Electric Circuit...

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